[aida] Superpowers: Climbing on the Stack in Aida/Web

Janko Mivšek janko.mivsek at eranova.si
Sat Sep 13 11:27:42 CEST 2008


Hi Travis,

http://smalltalk-superpowers.org, good idea indeed!

Here is my contribution from Aida/Web Smalltalk Web Application Server. 
Please feel free to correct grammar and also ask me for better 
explanation if you found something not clear enough. Also, I won't be at 
OOPSLA, so we need to find a presenter ...

"Climbing on the Stack" in AIDA/Web

AIDA/Web is web framework and application server where quite a few 
Smalltalk reflection techniques are used for maintaining the simplicity 
of the framework. One is finding a reference to the web session in 
context of which we are building a web page. To avoid having session 
parameter in every method call, the session can be found from the stack, 
by help of pseudo variable thisContest and we are calling this technique 
"climbing on the stack".

Here is the method (on Squeak):

   Object>>firstSessionFromStack

     | context |
     context := thisContext.
     [context notNil] whileTrue:
         [(context receiver isKindOf: AIDASite)
            ifTrue: [^context at: 3]
         context := context sender].
     ^nil

This method is trying to find a first sender up in stack which is an 
AIDASite and get a session reference from 3th parameter of the method 
call to the AIDASite, which happens to be always:

	#answer: anObject toGetOrPost: aRequest on: aWebSession

Of course this technique costs some time, so it is advised to use it 
sparingly, where there is not other possibility to get a session and 
where performance is not too important.


Best regards
Janko



-- 
Janko Mivšek
AIDA/Web
Smalltalk Web Application Server
http://www.aidaweb.si


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